U.S. Pat. No. 2,966,648 discloses an electric heating element that is comprising an elongated flexible flat high-temperature heating ribbon. The heating element is further comprising a tubular sheathing comprising fine high temperature resistant metal wires. The sheathing is provided around the heating ribbon. The fine high temperature resistant metal wires are provided to reinforce the heating element while providing a flexible heating element, they are not used as electrical conductor (they are electrically insulated from the heating ribbon) and are therefore not participating in the generation of heat in the electric heating element. The metal wires may be of any suitable corrosion and high-temperature resistance alloy, as, for example, stainless steel, Inconel, Nichrome or Kanthal.
Cables for heating applications that are comprising a multiple of metallic filaments as electrical conductors (and participating in the heat generation) are known. Cables for car seat heating are more and more widely applied in modern vehicles. Copper or copper alloy lacquered cables are used. The advantage of copper is its high specific electrical conductivity combined with a good plastic deformation. The disadvantage of copper is a low flex life, i.e. a low resistance to repeated bending cycles, and the limitation in electrical resistance range given the high electrical conductivity of copper.
Besides for car seat heating applications, heating cables are used for other applications, e.g. in garments.
In practice when using copper cables the range of electrical resistance is limited to 0.40 Ω/m (Ohm/meter), at maximum up to 0.50 Ω/m. The range between 0.50 Ω/m and 2.0 Ω/m is difficult, if not impossible, to reach. The resistance values that are indicated are resistance values at 20° C. Of course one could limit the number of filaments in the cable or reduce the diameter of the filaments in order to increase the electrical resistance. For example, a cable of twenty copper filaments with each a diameter of 50 μm has an electrical resistance of approximately 0.43 Ω/m (at 20° C.). This construction 20*50 μm is already at the lower limit regarding number of filaments and filament diameter and will give an unacceptably low strength and lifetime, especially a low flex life.
Alternatives are being provided by combining the good electrical conductivity of copper with the higher strength, higher flex life and higher electrical resistance of stainless steel. EP-A-1507904 discloses such a combination cable where stainless steel cores are provided with a copper coating. EP-A-1507905 discloses an alternative combination cable where stainless steel filaments are intertwined with copper filaments, both types of filaments are used as electrical conductors and are participating in the generation of heat. While offering advantages as to an increased flex life, these combination cables have the drawback of requiring at least two different materials, namely stainless steel and copper to obtain the required electrical resistance values and have the drawback that the range of electrical resistance is still too limited because of the high conductivity of copper.
A further drawback of existing heating cables is that the cable itself does not contain a safety function in case the heating cable gets overheated. There is a need for having heating cables that have self-regulating characteristics.